∵{an}为等比数列∴a5-a1=a1(q^4-1)=15……①a4-a2=a1(q³-q)=6……②①/②式得：(q^4-1)/(q³-q)=15/6化简得：2q²-5q+2=0(q-2)(2q-1)=0解得：q=2或1/21°当q=2时,a1=15/(q^4-1)=1则a3=a1q²=1*2&#...